Circuit NotesThe circuit transmits on Medium Wave (this is the small problem with the police). IC1a, together with a sensor (try a 20cm x 20cm sheet of tin foil) oscillates at just over 1MHz. This is modulated by an audio frequency (a continuous beep) produced by IC1b. When a hand or a foot approaches the sensor, the frequency of the transmitter (IC1a) drops appreciably.

Suppose now that the circuit transmits at 1MHz. Suppose also that your radio is tuned to a frequency just below this. The 1MHz transmission will therefore not be heard by the radio. But bring a hand or a foot near to the sensor, and the transmitter's frequency will drop, and a beep will be heard from the radio.

Attach the antenna to a multiplug adapter that is plugged into the mains, and you will find that the Medium Wave transmission radiates from every wire in your house. Now place a suitably tuned Medium Wave radio near some wires or a plug point in your house, and an early-warning system is set up.

Instead of using the sheet of tin foil as the sensor, you could use a doorknob, or burglar bars. Or you could use a pushbutton and series resistor (wired in series with the 33K resistor - the pushbutton would short it out) to decrease the frequency of IC1a, so activating the system by means of a pushbutton switch. In this case, the radio would be tuned to a frequency just below that of the transmitter. Schematics for Radio Wave Alarm Circuit Electronics

This is basically a crystal radio with an audioamplifier which is fairly sensitive and receives several strong stations in the Los Angeles area with a minimal 15 foot antenna. Longer antennas will provide a stronger signal but the selectivity will be worse and strong stations may be heard in the background of weaker ones. Using a long wire antenna, the selectivity can be improved by connecting it to one of the taps on the coil instead of the junction of the capacitor and coil. Some connection to ground is required but I found that standing outside on a concrete slab and just allowing the long headphone leads to lay on the concrete was sufficient to listen to the local news station (KNX 1070). The inductor was wound with 200 turns of #28 enameled copper wire on a 7/8 diameter, 4 inch length of PVC pipe, which yields about 220 uH. The inductor was wound with taps every 20 turns so the diode and antenna connections could be selected for best results which turned out to be 60 turns from the antenna end for the diode. The diode should be a germanium (1N34A type) for best results, but silicon diodes will also work if the signal is strong enough. The carrier frequency is removed from the rectified signal at the cathode of the diode by the 300 pF cap and the audio frequency is passed by the 0.1uF capacitor to the non-inverting input of the first op-amp which functions as a high impedance buffer stage. The second op-amp stage increases the voltage level about 50 times and is DC coupled to the first through the 10K resistor. If the pairs of 100K and 1 Meg resistors are not close in value (1%) you may need to either use closer matched values or add a capacitor in series with the 10K resistor to keep the DC voltage at the transistor emitter between 3 and 6 volts. Another approach would be to reduce the overall gain with a smaller feedback resistor (470K). High impedance headphones will probably work best, but walkman stereo type headphones will also work. Circuit draws about 10 mA from a 9 volt source. Germanium diodes (1N34A) types are available from Radio Shack, #276-1123.

Q1 = MFE201, SK3991, or NTE454. N-Channel, dual-gate MOSFET (see text)Q2 = 2SC2570, NTE10, NTE107. NPN VHF/UHF silicon transistor (see text)Note: If you use the NTE107 as a replacement, make sure to insert it correctly into the pcb. The orientation is different than as shown on the parts layoutdiagram. (e-c-b seen front view for NTE107). See this Data Sheet

Signals from the microphone in the fm transmitter is reinforced by Q1, Q2 with carrier frequency generator is determined by the C5 and L1. The frequency of the FM transmitter is in the range 80 MHz - 108 MHz. L1 can be made ​​with as many as 24 e-mail wire wrap and 6 wrap. The following is a picture series for the fm transmitter fm transmitter referred to in article 2 of this transistor.

This fm transmitter antenna is connected to the mid point of the antenna length L1 and preferably between 8-12 inches. FM transmitter is only used for experiment and learning materials are not to be used for day-to-day, because the use of FM transmitter frequency regulated and protected by law may be understandable.

1. The input signalThe input signal in the form of electrical signals generated by mechanical equipment modifier into electrical vibrations. Tools that generate these signals include a microphone, LPs, and others. Power signals issued by these tools, the amplitude is still too small, so it requires strengthening again.

3. Oscillator circuitOscillator circuit is an electronic circuit that functions to produce high vibration. The frequency of the resulting circuit is high because if low, this electrical vibration will not radiate much.Oscillator which is used in the transmitter is the oscillator RF. RF oscillator is a high frequency generator that serves as a signal carrier. Terms oscillator which is capable of generating both high-frequency equipment. Electronic components that can generate these frequencies is Crystal. Crystals are commonly used in FM transmitter has rangkainan 27MHz output frequency.

4. Buffer amplifier (Buffer)Buffer serves to insulate the RF oscillator with power amplifier, so the fixed oscillator frequency. In addition, this section also reinforces the amplitude of RF signals.

5. ModulatorModulator is part of an audio signal / information signal with carrier signal. In AM transmitters, the carrier signal amplitude changes are made in accordance with changes in signal amplitude information.

6. Power amplifierServed to strengthen the power amplifier before dikirimkam termodulsi signal to the antenna to be transmitted. Type of amplifier used is a class C power amplifier class C power amplifier diplih because of its ability to strengthen the radio frequency (above 20kHz). Transistors are used in this amplifier is the RF power transistor, which has the characteristics of the output power ranged from 1 to 75 watts.

8. AntennaAntennas in a transmitter is spearheading the delivery of a radio signal transmitter to a receiver. If the antenna is used at a transmitter not meeting the correct specification, then the results are not expected to be achieved. In addition to the range to be erratic also the possibility of damage to the transmitter circuit due to back pressure from the antenna. When the rod antenna is widely used on aircraft such as the transistor radio, car radio receiver, walky Talky, Handy Talky, and so forth.

How it works series AM transmitters:

This powerful AM transmitters ntuk fairly large, use the tuner 3.587 mhz ceramic resonator and resonator filters are also sold with a value of 5.5 mhz, 7.7 mhz and 10.7 mhz.Jarak transmitter range is approximately 2-4 km. the working principle of this circuit are: filter resonator / ceramic filters generate value from the filter frequency resonator tsb. This frequency is amplified by the transistor can be changed so that t1.frekuensi dpperlukan C7 as a regulator / placement. superimposed sound signal through the audio transformer.frekuensi which awakened by the filter resonator and fed t1 kepenguat to be strengthened further so as to achieve the desired power passed to the antenna. t2 and t3 as a buffer as a final power amp Schematics for AM transmitter circuit analysis Circuit Electronics

Here's a simple VHF FM transmitter that could be used to play audio files from an MP3 player or computer on a standard VHF FM radio. The circuit use no coils that have to be wound. This FM transmitter can be used to listen to your own music throughout your home. When this FM transmitter used in the car, there is no need for a separate input to the car stereo to play back the music files from your MP3 player.

To keep the circuit simple as well as compact, it was decided to use a chip made by Maxim Integrated Products, the MAX2606 [1]. This IC from the MAX2605-MAX2609 series has been specifically designed for low-noise RF applications with a fixed frequency. The VCO (Voltage Controlled Oscillator) in this IC uses a Colpitts oscillator circuit. The variable-capacitance (varicap) diode and feedback capacitors for the tuning have also been integrated on this chip, so that you only need an external inductor to fix the central oscillator frequency.

It is possible to fine-tune the frequency by varying the voltage to the varicap. Not much is demanded of the inductor, a type with a relatively low Q factor (35 to 40) is sufficient according to Maxim. The supply voltage to the IC should be between 2.7 and 5.5 V, the current consumption is between 2 and 4 mA. With values like these it seemed a good idea to supply the circuit with power from a USB port.

A common-mode choke is connected in series with the USB connections in order to avoid interference between the circuit and the PC supply. There is not much else to the circuit. The stereo signal connected to K1 is combined via R1 and R2 and is then passed via volume control P1 to the Tune input of IC1, where it causes the carrier wave to be frequency modulated. Filter R6/C7 is used to restrict the bandwidth of the audio signal. The setting of the frequency (across the whole VHF FM boadcast band) is done with P2, which is connected to the 5 V supply voltage.

The PCB designed uses resistors and capacitors with 0805 SMD packaging. The size of the board is only 41.2 x 17.9 mm, which is practically dongle-sized. For the aerial an almost straight copper track has been placed at the edge of the board. In practice we achieved a range of about 6 metres (18 feet) with this. There is also room for a 5-way SIL header on the board. Here we find the inputs to the 3.5 mm jack plug, the input to P1 and the supply voltage. The latter permits the circuit to be powered independently from the mains supply, via for example three AA batteries or a Lithium button cell. Inductor L1 in the prototype is a type made by Murata that has a fairly high Q factor: minimum 60 at 100 MHz.

Layout PCB USB FM transmitter

Take care when you solder filter choke L2, since the connections on both sides are very close together. The supply voltage is connected to this, so make sure that you don’t short out the USB supply! Use a resistance meter to check that there is no short between the two supply connectors before connecting the circuit to a USB port on a computer or to the batteries.

P1 has the opposite effect to what you would expect (clockwise reduces the volume), because this made the board layout much easier. The deviation and audio bandwidth varies with the setting of P1. The maximum sensitivity of the audio input is fairly large. With P1 set to its maximum level, a stereo input of 10 mVrms is sufficient for the sound on the radio to remain clear. This also depends on the setting of the VCO. With a higher tuning voltage the input signal may be almost twice as large (see VCO tuning curve in the data sheet). Above that level some audible distortion becomes apparent. If the attenuation can’t be easily set by P1, you can increase the values of R1 and R2 without any problems.

Measurements with an RF analyzer showed that the third harmonic had a strong presence in the transmitted spectrum (about 10 dB below the fundamental frequency). This should really have been much lower. With a low-impedance source connected to both inputs the bandwidth varies from 13.1 kHz (P1 at maximum) to 57 kHz (with the wiper of P1 set to 1/10).

In this circuit the pre-emphasis of the input is missing. Radios in Europe have a built-in de-emphasis network of 50 μs (75 μs in the US). The sound from the radio will therefore sound noticeably muffled. To correct this, and also to stop a stereo receiver from mistakenly reacting to a 19 kHz component in the audio signal, an enhancement circuit Is published elsewhere in this issue (Pre-emphasis for FM Transmitter, also with a PCB). Author: Mathieu Coustans, Elektor Magazine, 2009

Wireless FM transmitter circuit Microphone Hi Fi using a model-ready transmitters, this is because the buil-up transmitter module has a transmission capability of a stable and very effective to emit 10 mW RF. so that the target of Hi Fi FM Wireless Microphone transmitter circuit Hi Fi can be realized. The series can be seen from the FM transmitter Microphone Wireless Hi Fi transmitter module is that it stands alone with an additional R7, R8 and C6 to produce high-quality RF frequency.

Input section of the circuit transmitter Wireless FM Microphone Hi Fi signals to the modulation of 100mV RMS. A microphone preamplifier with IC TL081 is used to reproduce the signal from the microphone into a signal with 100mV rms level as needed this wireless FM transmitter module. strengthening the signal from the microphone preamplifier circuit in FM transmitter Microphone Wireless Hi Fiini regulated by P1. R1, R2 and C1 in the range of an FM transmitter Microphone Wireless Hi Fi is used to provide supply to the microphone electred used. Schematics for Wireless transmitter microphone circuit Circuit Electronics